I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to improving reverse link capacity in a radiotelephone system.
II. Description of the Related Art
Multiple access techniques are designed to make efficient use of the limited radio frequency spectrum. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA wireless technology, governed by Electronic Industry Association/Telecommunication Industry Association Interim Specification-95 (IS-95), employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only part of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, is affected less by the fade condition than a receiver using other types of signals.
The spread spectrum technique is accomplished by modulating each base band data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques are found in M.K. Simon, Spread Spectrum Communications, Volume I, pp. 262-358.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously on the same frequency. A particular receiver then determines which signal is intended for that receiver by the unique spreading code in each signal. The signals at that frequency, without the particular spreading code intended for that particular receiver, appear to be noise to that receiver and are ignored.
Since multiple radiotelephones and base stations transmit on the same frequency, power control is an important component of the CDMA modulation technique. A higher power output by a radiotelephone and/or base station increases its signal quality but also increases the interference experienced by the other radiotelephones and base stations in the system. In order to keep the radiotelephones and base stations from transmitting at too much power, thereby decreasing system capacity, some form of power control must be implemented.
The radiotelephone can aid the base station in the control of the power on the forward link (from the base station to the radiotelephone) by feedback on the reverse link (from the radiotelephone to the base station). This is accomplished by either a power control message that is sent when appropriate thresholds are triggered or an erasure indicator bit on a reverse link frame that indicates the status of a previously sent forward link frame. The base station may then adjust its power level to the specific user accordingly. This is referred to in the art as forward link power control.
The ratio .sup.E.sub.b /.sub.N.sub..sub.o is a standard quality measurement for digital communications system performance. The ratio expresses the bit-energy-to-noise-density of the received signal. .sup.E.sub.b /.sub.N.sub..sub.o can be considered a metric that characterizes the performance of one communication system over another similar conditions; the smaller the required .sup.E.sub.b /.sub.N.sub..sub.o for a given grade of service, the more efficient is the system modulation and detection process. A more detailed discussion of this concept can be seen in B. Sklar, Digital Communications, Fundamentals and Applications, Chapter 3 (1988).
Each user on the reverse link is an interferer to the other users. Increasing the .sup.E.sub.b /.sub.N.sub..sub.o of a particular user translates to an increase in the user's received bit energy (E.sub.b). This causes an increased noise density (N.sub.o) that other users detect.
Therefore, it is essential to allocate each user or reverse traffic channel the required amount of .sup.E.sub.b /.sub.N.sub..sub.o to achieve and sustain the reverse link at a given frame error rate (FER) or grade of service (GOS). Providing less than the required amount increases the drop call probabilities and FERs. Allocating an excessive amount of .sup.E.sub.b /.sub.N.sub..sub.o results in a reduction in reverse link network capacity and coverage.
The required .sup.E.sub.b /.sub.N.sub..sub.o is dynamic in that it depends on the mobile velocity, the fading characteristics, the multi-path environment, and the soft/softer hand-off status. Therefore, a dynamic reverse link power control process for controlling the required .sup.E.sub.b /.sub.N.sub..sub.o is essential to maximize reverse link network capacity.
A typical prior art reverse link power control process is illustrated in FIG. 1. This process is used by the base station in determining whether to transmit a power up or power down command to the radiotelephone.
The base station receives information from the radiotelephone. This information is in the form of a stream of data frames. The type and format of this data is well known in the art.
The base station demodulates (step 101) the information. The .sup.E.sub.b /.sub.N.sub..sub.o is measured (step 105), for a power control group duration (a sixteenth of a frame). The measured .sup.E.sub.b /.sub.N.sub..sub.o is compared (step 115) to a target .sup.E.sub.b /.sub.N.sub..sub.o . The target .sup.E.sub.b /.sub.N.sub..sub.o is adjusted on a frame by frame basis. The target is modified (step 110) in such a way as to maintain the required frame error rate (FER). The network operators typically set the FER target.
In order to modify the target .sup.E.sub.b /.sub.N.sub..sub.o , the quality of each received frame is determined (step 125). If a particular received frame was good, the target is decreased a predetermined amount. If the particular received frame was bad, the target is increased a predetermined amount. The network operators set the predetermined amount. The principles behind determining the quality of the frames is well known in the art and is discussed in John G. Proakis, Digital Communications, Chapter 7.
If the measured .sup.E.sub.b /.sub.N.sub..sub.o is less than the target .sup.E.sub.b /.sub.N.sub..sub.o , the base station instructs (step 120) the radiotelephone to power up by a predetermined amount, typically 1.0 dB. If the measured .sup.E.sub.b /.sub.N.sub..sub.o is greater than the target .sup.E.sub.b /.sub.N.sub..sub.o , the radiotelephone instructs (step 120) the base station to power down by a predetermined amount, typically 1.0 dB.
The reverse link power control process is based on an outer loop process and an inner loop process. The outer loop process corrects the .sup.E.sub.b /.sub.N.sub..sub.o target requirement based on the performance of the reverse traffic channel on a frame by frame basis. Frame rate and quality determination is based on the cyclic redundancy code (CRC) employed and associated algorithms. Each time a frame is received, a CRC check is performed four times, once for each frame rate. If all the CRC's fail, the frame is labeled an erasure (bad frame). If one passes, the frame rate is determined as the rate corresponding to the CRC check. If more than one passes, the CRC check yielding the lowest symbol error rate is used as an indication of the frame rate.
The outer loop process is a function of the required FER. As the radiotelephone moves, the .sup.E.sub.b /.sub.N.sub..sub.o required to maintain communication to a given FER changes. If a frame is good, the target .sup.E.sub.b /.sub.N.sub..sub.o is reduced marginally. This .sup.E.sub.b /.sub.N.sub..sub.o reduction continues in steps for each good frame received.
If a bad frame is received, the environment must have changed requiring a higher .sup.E.sub.b /.sub.N.sub..sub.o . In this case, the target .sup.E.sub.b /.sub.N.sub..sub.o is increased by a large step in order to reduce the chances of not increasing the target enough. If the target is now larger, the measured .sup.E.sub.b /.sub.N.sub..sub.o is less per power control group and the base station(s) sends power up commands to the radiotelephone. Assuming the larger target .sup.E.sub.b /.sub.N.sub..sub.o is sufficient, good frames result and the process slowly steps down the target .sup.E.sub.b /.sub.N.sub..sub.o again. The up and down step sizes are coupled mathematically to meet the required FER target set by operators.
The inner loop power control process estimates or measures the .sup.E.sub.b /.sub.N.sub..sub.o over one power control group of 1.25 ms. The measured .sup.E.sub.b /.sub.N.sub..sub.o is compared to the target .sup.E.sub.b /.sub.N.sub..sub.o . Based on the comparison, a power up or power down command is sent by the base station(s) to the radiotelephone on the forward traffic channel.
The reverse link power control process discussed above attempts to track the change in .sup.E.sub.b /.sub.N.sub..sub.o requirements as the mobile velocity, fading conditions, and multipath profile change. This prior art reverse link power control process fails to operate efficiently when the frame rate changes.
Each frame rate is designed to operate at a different target FER. For example, full rate frames typically operate at 1-2% FER targets and eighth rate frames typically operate at 5% FER. The eighth rate frames operate at a higher FER because they can do this without affecting voice quality. As a result, this should provide a higher reverse link capacity; i.e., a lower .sup.E.sub.b /.sub.N.sub..sub.o means a higher capacity and of course higher FER. The full rate frames operate at a lower FER since any higher FERs would affect the voice quality of the system. The different targets result in different .sup.E.sub.b /.sub.N.sub..sub.o requirements under similar radio frequency conditions. Even for the same target FER, the .sup.E.sub.b /.sub.N.sub..sub.o requirement may be different for different rates due to different speeds of power control at the different rates.
The time in a particular rate before a rate change is significant since the reverse link power control process requires a certain time to correct for the different .sup.E.sub.b /.sub.N.sub..sub.o requirements. For example, the .sup.E.sub.b /.sub.N.sub..sub.o requirement for the eighth rate may be 1.5-2.0 dB less than the full rate. Therefore, when the rate changes from eighth to full rate, the current .sup.E.sub.b /.sub.N.sub..sub.o is initially insufficient to meet the 1% FER for full rate. Similarly, when the rate changes from full rate to eighth rate, the resulting .sup.E.sub.b /.sub.N.sub..sub.o is initially more than sufficient to meet the 5% FER.
If the radiotelephone remained in a particular rate for a long enough period of time, this effect would be averaged out and the FER targets would be met. However, the duration of frames in the different rates is bursty and small, resulting in a performance degradation. The end result is that the radiotelephone is unable to meet a 1% full rate FER target and a 5% eighth rate FER target. Additionally, RF conditions changing complicate this process. There is a need for a power control process that can rapidly update reverse link .sup.E.sub.b /.sub.N.sub..sub.o requirements during rate changes.